Radiation includes a great variety of rays and particles that have tremendously different energies. Within this spectrum is a group of rays and particles with enough energy to fragment atoms and molecules into ions and free radicles; this is ionizing radiation. Radiation with extremely high energy goes through matter with little reaction, e.g., pions and nutrinos traverse the earth with little effect. Other radiation has too little energy to disrupt molecules under usual conditions; visible light, most UV radiation, microwaves, infrared rays, and other waves may excite molecules and dissociate ions, but they do not disrupt molecules without extreme heat. Mixed radiation sources such as cosmic rays include secondary radiation generated when galactic rays hit the earth’s atmosphere. Ionizing radiation includes electromagnetic rays; X- and γ rays are distinguished only by their source, extranuclear and nuclear, respectively. X-rays usually have less energy than y rays. Ionizing radiation includes particles, such as α rays, which are helium nuclei, β rays and electron beams from disintegrating atoms and accelerators; neutron beams; positron beams; and a variety of ions and atoms that have sufficiently high velocity to fragment atoms and molecules with which they collide.